Pumps are often utilized to meter or otherwise deliver small quantities of fluid with a required high precision. Such accurate and repeatable dispensing of a fluid is often required in laboratory instrumentation environments such as the photographic processing industry or in the medical field such as in the metering and delivery of a low volume, precise amount of reagent to test blood.
Many pumps used for this purpose are of the positive displacement type which normally include poppet valves or check valves at the inlets and outlets thereof. However, such valves are usually, most conveniently, made of rubber material which can be the subject of attack by many chemicals. As a result, such valves will deteriorate causing the pump to lose its accuracy and eventually resulting in the need for replacement.
Thus, valveless, positive displacement, piston pumps are more suited for this application. However, known of such pumps may not consistently provide the accuracy required for many applications. For example, the positive displacement piston pump shown in U.S. Pat. No. 3,168,872 is typical of those that are available today. The problem with these types of pumps is that there is some dead space in the piston chamber where a small amount of fluid can remain after each piston stroke. Since most all fluids contain entrapped gas, such may also tend to accumulate in that dead space and form a small gas bubble. Eventually, the piston which is intended to deliver fluid will be compressing gas and not dispensing the correct amount of fluid. In effect then, the stroke of the piston is compressing and uncompressing the gas bubble to the detriment of accurate volume fluid dispensing.
The need exists, therefore, for a pump which will repeatedly deliver a precise amount of fluid, even in small microliter volumes.